1. Technical Field
The present invention is directed toward improving output of cellular communication units, and more particularly toward improving the quality of channel estimates used to convert received signals to usable output in cellular communication units.
2. Background Art
Signals for wireless systems are subjected to varying conditions which can degrade the signal received by the mobile units or mobile stations (xe2x80x9cMSxe2x80x9d) using the system. Such conditions can also degrade the signal received by base transceiver stations (xe2x80x9cBTSxe2x80x9d) from mobile stations.
For example, MS and BTS can receive signals from multiple directions (e.g., a specific signal can be received by an MS directly from a BTS, and reflected off of many different ground objects), with the varying signal sources potentially being out of phase and thereby tending to cancel each other out to some degree, reducing signal strength. Such signal fading, generally known as Rayleigh fading, occurs spatially over the area of the system, with specific areas potentially having significant fading which could cause the mobile unit to lose the signal entirely.
The net result of such factors is that the signal which is transmitted by the transmitter (e.g., a cell tower) will be distorted by the time it reaches the receiver (e.g., a cellular telephone). In a cellular telephone call, for example, this can result in distortion objectionable to the ear, or even a lost signal.
In order to account for this distortion, channel estimates have been used to determine the signal distortion at known pilot symbols in the data bursts and channel coefficients (correction factors used to derive channel estimates) at other symbols in the data bursts have been interpolated based on the channel estimates at the pilot symbols. As an example, data bursts have been transmitted in the IS-136 System with 162 symbols, each symbol comprising two bits. In a proposed extension of the IS-136 System, the data bursts of 162 symbols at predetermined, known locations Pi in the data bursts are predetermined, known pilot symbols SPi (where i=1 to n, n being the number of pilot symbols used). In the proposed extension of the IS-136 System, each symbol contains three bits.
The channel coefficients determined from the pilot symbols have been used to estimate the most likely value for each data symbol in a data burst. That is, the channel coefficients determined from the pilot symbols have been interpolated to determine the channel coefficients at the other symbols (i.e., data symbols) in the data burst by using an interpolator or filter suited to work under the conditions most likely to be encountered by the communication unit.
A more detailed discussion of the prior art use of channel estimates is included below in context with the Description of the Preferred Embodiment. While such prior art channel estimates have enabled the received signal to be more accurately demodulated to provide improved output, such channel estimates are still susceptible to error and resulting degradation of the output.
The present invention is directed toward overcoming one or more of the problems set forth above.
In one aspect of the present invention, a transceiver adapted to communicate via signals with another transceiver is provided. The one transceiver includes a receiver adapted to receive from the other transceiver a signal having multiple symbols therein including predetermined pilot symbols, and a transmitter adapted to transmit signals having multiple symbols therein including power control bits, the power control bits instructing the other transceiver to establish selected power levels for its signal to the receiver. The transceiver also includes a processor adapted to interpolate channel coefficients for received symbols as a function of both (1) the selected power levels of the other transceiver signal and (2) the difference between the predetermined pilot symbols and the pilot symbols as received by the receiver. A demodulator demodulates all received symbols based on the interpolated channel coefficients.
In one preferred form, pilot symbol channel coefficients CPi are determined by the processor so as to minimize the following summation for selected pilot symbols:
xcexa3E{|RPixe2x88x92CPi*SPi*{square root over (W)}Pi|2}
where: E is the expectation value for the power level at pilot position Pi,
WPi is the known power level at pilot position Pi,
RPi is the pilot symbol as received at pilot position Pi, and
SPi is the pilot symbol known to have been sent at pilot position Pi.
With this form, the processor includes an interpolation filter which interpolates channel coefficients for other than the pilot symbol channel coefficients based on the determined pilot symbol channel coefficients CPi.
In a second preferred form, the processor is adapted to calculate an interpolation filter as a function of the selected power levels, and channel coefficients for received symbols other than the pilot symbols are interpolated using the calculated interpolation filter.
In this second preferred form, when the transceiver is subjected to conditions in which the power levels can be set accurately and the power control bits will be received correctly by the other transceiver, the processor uses the following auto-correlation function Rc, in calculating the interpolation filter:
Rcxe2x80x2|WiWk(i,k)={square root over (W)}i{square root over (W)}kJO(2xcfx80fd(ixe2x88x92k)Ts)
where: JO is the Bessel function of the first kind;
fD is the Doppler spread of the channel;
Ts is the symbol duration, and
Wi and Wk are known power levels at positions i and k in the received signal.
Alternatively, in this second preferred form, when the transceiver is subjected to conditions in which the power control bits are always received correctly and changes in the power level are independent and identically distributed, the-processor uses the following auto-correlation function Rc, in calculating the interpolation filter:
                              R                                                                      c                  xe2x80x2                                ⁢                                  xe2x80x83                                ⁢                                  (                                      i                    ,                    k                                    )                                            |                              y                1                                      ,                          y              2                        ,                          xe2x80x83                        ⁢            …            ⁢                          xe2x80x83                        ,                          y              L                                      =                  xe2x80x83                ⁢                              E            ⁡                          [                              W                i                            ]                                ⁢                      xe2x80x83                    ⁢                                    E              ⁡                              [                                                      s                    1                                                  ]                                                                    y                1                            ,                              y                2                            ,                              xe2x80x83                            ⁢              …              ⁢                              xe2x80x83                            ,                              y                L                                              ⁢                      xe2x80x83                    ⁢                      J            o                    ⁢                      xe2x80x83                    ⁢                      (                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                D                            ⁢                              xe2x80x83                            ⁢                              (                                  i                  -                  k                                )                            ⁢                              xe2x80x83                            ⁢                              T                s                                      )                                                  =                  xe2x80x83                ⁢                                            AE              ⁡                              [                                                      s                    1                                                  ]                                                                    y                1                            ,                              y                2                            ,                              xe2x80x83                            ⁢              …              ⁢                              xe2x80x83                            ,                              y                L                                              ⁢                      xe2x80x83                    ⁢                      J            o                    ⁢                      xe2x80x83                    ⁢                      (                          2              ⁢                              xe2x80x83                            ⁢              π              ⁢                              xe2x80x83                            ⁢                              f                D                            ⁢                              xe2x80x83                            ⁢                              (                                  i                  -                  k                                )                            ⁢                              xe2x80x83                            ⁢                              T                s                                      )                              
where: JO is the Bessel function of the first kind;
fD is the Doppler spread of the channel;
Ts is the symbol duration;
i and k are positions in the received signal;
Pi is the known power level at position i in the received signal;
s1, is the incremental change in signal power resulting from a non-zero power control bit;
yi is either 1 or xe2x88x921, where at position k there have been L incremental power steps;
E is expectation;
Wi is the known power level at positions i of the received signal; and
A is the average power level of the received signal.
In another alternative of the second preferred, form, when the transceiver is subjected to conditions in which the transmitted power control bits are received by the other transceiver with very poor signal to noise ratio (SNR) and incremental changes in the power level of the received signal are accurate, the processor uses the following auto-correlation function Rcxe2x80x2 in calculating the interpolation filter:             R              c        xe2x80x2              ⁡          (              i        ,        k            )        =                    AJ        0            ⁡              (                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                                    f              D                        ⁡                          (                              i                -                k                            )                                ⁢                      T            s                          )              ⁢                  (                                            s                        +                                          s                                            -                1                                              2                )            L      
where: JO is the Bessel function of the first kind;
fD is the Doppler spread of the channel;
Ts is the symbol duration;
i and k are positions in the received signal;
s is the incremental change xcex94Wi in signal power resulting from a non-zero power control bit;
L is the number of incremental power steps; and
A is the average power level of the received signal.
In still another alternative of the second preferred form, when the transceiver is subjected to conditions in which the power control bits are received with poor signal to noise ratio (SNR) and incremental changes in the power level of the received signal are not accurate, the processor uses the following auto-correlation function Rcxe2x80x2 in calculating the interpolation filter:             R              c        xe2x80x2              ⁡          (              i        ,        k            )        =                    AJ        0            ⁡              (                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                                    f              D                        ⁡                          (                              i                -                k                            )                                ⁢                      T            s                          )              ⁢                  (                                            E              ⁡                              [                                  s                                ]                                      +                          E              ⁡                              [                                                      s                                                        -                    1                                                  ]                                              2                )            L      
where: JO is the Bessel function of the first kind;
fD is the Doppler spread of the channel;
Ts is the symbol duration;
i and k are positions in the received signal;
s is the incremental change in signal power resulting from a non-zero power control bit;
L is the number of incremental power steps;
E is expectation; and
A is the average power level of the received signal.
In another aspect of the first form of the present invention, a transceiver is adapted to communicate via signals with another transceiver, and includes a receiver, transmitter and processor. The receiver is adapted to receive from the other transceiver a signal having multiple symbols therein including predetermined pilot symbols. The transmitter is adapted to transmit signals having multiple symbols therein including power control bits, the power control bits instructing the other transceiver to establish selected power levels for its signal to the receiver. The processor is adapted to (a) determine channel coefficients for the pilot symbols as a function of both (1) the selected power levels of the other transceiver signal and (2) the difference between the predetermined pilot symbols and the pilot symbols as received by the receiver, (b) interpolate channel coefficients for received symbols other than the pilot symbols based on the channel coefficients determined for the pilot symbols, and (c) demodulate all received symbols based on the interpolated channel coefficients.
The one transceiver may be a base station and the other transceiver a mobile station of a cellular telephone system, or vice versa.
In a preferred construction of this aspect of the first form of the present invention, the pilot symbol channel coefficients CPi are determined so as to minimize the following summation for selected pilot symbols:
xcexa3E{|RPixe2x88x92CPi*SPi*{square root over (W)}Pi|2}
where: E is the expectation value for the power level at pilot position Pi,
WPi is the known power level at pilot position Pi,
RPi is the pilot symbol as received at pilot position Pi, and
SPi is the pilot symbol known to have been sent at pilot position Pi.
In another preferred embodiment of this aspect of the first form of the present invention, the processor includes an interpolation filter, and the interpolation filter uses the determined channel coefficients for the pilot symbols to interpolate the channel coefficients for received symbols other than the pilot symbols.
In still another aspect of the first form of the present invention, a transceiver is adapted to communicate with another transceiver via signals having multiple symbols therein, where the one transceiver adapted to transmit outgoing signals with power control bits instructing the other transceiver to establish selected power levels in its signal and the other transceiver adapted to transmit return signals to the one transceiver at the selected power levels with symbols including predetermined pilot symbols. The one transceiver includes a transmitter adapted to transmit the outgoing signals, a receiver adapted to receive the return signals from the other transceiver, memory storing the predetermined pilot symbols, and a processor. The processor is adapted to (a) determine channel coefficients for the pilot symbols as a function of both (1) the selected power levels of the other transceiver signal and (2) the difference between the predetermined pilot symbols stored in the memory and the pilot symbols as received by the receiver, (b) interpolate channel coefficients for symbols in the based on channel coefficients determined for the pilot symbols, and (c) demodulate received symbols based on the interpolated channel coefficients.
The above described alternates and preferred constructions of the first form of the present invention may also be used with this aspect of the first form of the present invention.
In another aspect of the second form of the present invention, a unit for communicating with a transceiver which transmits return signals in data bursts having data symbols and predetermined pilot symbols in a plurality of slots is provided, including a transmitter adapted to send outgoing signals in data bursts to the transceiver, the data bursts of the outgoing signals having a plurality of slots including data symbols comprising power control bits instructing the transceiver on the power to use when transmitting a return signal back to the unit, and a receiver adapted to receive a return signal from the transceiver. The unit also includes a memory storing (a) the predetermined pilot symbols, (b) an auto-correlation function based at least in part upon the power levels of the return signal when transmitted by the transceiver, and (c) an algorithm for calculating an interpolator based on the auto-correlation function. A comparator is adapted to determine the difference between the pilot symbols in the return signal as received by the receiver and the predetermined pilot symbols in the memory. The units further includes a processor adapted to determine channel coefficients for the received pilot symbols based on the difference determined by the comparator, use the algorithm to calculate an interpolation filter, and use the interpolation filter and the determined pilot symbol channel coefficients to derive data symbols in the received return signal. A unit demodulator is adapted to demodulate derived data symbols, and an output is adapted to receive symbols demodulated by the processor.
The unit may be a base station and the transceiver a mobile station of a cellular telephone system, or vice versa.
The above described alternates of the second form of the invention, including the use of different auto-correlation functions based on different conditions, may also be used with this aspect of the second form of the present invention.
In still another aspect of the second form of the present invention, a transceiver adapted to communicate via signals with another transceiver is provided. The one transceiver includes a receiver adapted to receive from the other transceiver a signal having multiple symbols therein including predetermined pilot symbols, and a transmitter adapted to transmit signals having multiple symbols therein including power control bits, the power control bits instructing the other transceiver to establish selected power levels for its signal to the receiver. The transceiver also includes a processor adapted to (a) calculate an interpolation filter based on the selected power levels of the other transceiver signal, (b) use the calculated interpolation filter to interpolate channel coefficients for received symbols other than the pilot symbols based on the difference between the predetermined pilot symbols and the pilot symbols as received by the receiver, and (c) demodulate all received symbols based on the interpolated channel coefficients.
The above described alternates of the second form of the invention, including the use of different auto-correlation functions based on different conditions to calculate the interpolation filter, may also be used with this aspect of the second form of the present invention.
In still another aspect of the present invention, a method for improving the signal reception of a communication unit is provided, comprising the steps of (a) transmitting a signal from a first transceiver with data symbols, selected ones of the data symbols having power control bits instructing a second transceiver to send a return signal at selected power levels; (b) receiving the transmitted signal at the second transceiver; (c) transmitting a return signal from the second transceiver with data symbols and pilot symbols, the pilot symbols having predetermined values at predetermined locations in the signal; (d) receiving the return signal at the first transceiver; (e) determining the return signal data symbols based at least in part on the instructions of the power control bits regarding the power level of the return signal; and (f) outputting the received signal using the data symbols corrected in step (e).
The first form of the invention may be used with this aspect of the present invention, with step (e) comprising (e1) determining correction factors at the predetermined locations of the pilot symbols in the return signal based on both (1) a comparison of the pilot symbols as received by the first transceiver with the predetermined values of the pilot symbols and (2) the instructions of the power control bits regarding the power level of the return signal, (e2) interpolating correction factors for data symbols based on the correction factors for the pilot symbols determined in step (e1), and (e3) correcting the received return signal data symbols using the correction factors interpolated in step (e2).
In a preferred alternate of this first form of the invention, correction factors CPi are determined in step (e1) by minimizing the following summation for selected pilot symbols:
xcexa3E{|RPixe2x88x92CPi*SPi*{square root over (W)}Pi|2}
where: E is the expectation value for the power level at pilot position Pi,
WPi is the known power level at pilot position Pi,
RPi is the pilot symbol as received at pilot position Pi, and
SPi is the pilot symbol known to have been sent at pilot position Pi.
The second form of the invention may also be used with this aspect of the present invention, with step (e) comprising (e1xe2x80x2) calculating an interpolation filter based at least in part on the selected power levels of the return signal, (e2xe2x80x2) determining correction factors for the pilot symbols, (e3xe2x80x2) interpolating correction factors for received return signal data symbols using the calculated interpolation filter and the determined pilot symbol correction factors, and (e4xe2x80x2) using the interpolated correction factors to determine the return signal data symbols.
In a preferred alternate of this second form of the invention, the calculating of step (e1xe2x80x2) uses an auto-correlation function which is based in part on the selected power levels of the return signal.
It is an object of the present invention to provide improved signal interpolation for a communication system such as a cellular telephone system.